Pointer class variable returns NULL from muncion - c++

Question

I'm developing a DLL (in Visual Studio 2013) to read TIFF (satellite images), using GDAL library, and having an issue to get back my variable with data - currently it empty (returns NULL).
In my DLL I have my funcion defined in the "RasterFuncs.h" like this:
namespace RasterFuncs
{
// This class is exported from the RasterFuncs.dll
class MyRasterFuncs
{
public:
// Open a raster file
static RASTERFUNCS_API int Open(char* rname, GDALDataset *poDataset);
};
}
and in my DLL cpp I have the following:
namespace RasterFuncs
{
int MyRasterFuncs::Open(char* rname, GDALDataset *poDataset)
{
poDataset = (GDALDataset *) GDALOpen(rname, GA_ReadOnly);
if (poDataset != NULL)
{
cout << "RasterXSize 1:" << poDataset->GetRasterXSize() << endl;
cout << "RasterYSize 1:" << poDataset->GetRasterYSize() << endl;
cout << "RasterCount 1:" << poDataset->GetRasterCount() << endl;
}
return 0;
}
}
at this point I have the poDataset with all image data.
However, I call this DLL form another CPP using the following code:
rfileName = "C:/Image1.tif";
// Open raster satelitte image
GDALDataset *poDataset = NULL;
GDALAllRegister();
RasterFuncs::MyRasterFuncs::Open(rfileName, poDataset);
if (poDataset != NULL)
{
cout << "RasterXSize:" << poDataset->GetRasterXSize() << endl;
cout << "RasterYSize:" << poDataset->GetRasterYSize() << endl;
cout << "RasterCount:" << poDataset->GetRasterCount() << endl;
}
and when I test poDataset that come back, shows NULL.
Anybody could help in this issue?
Thanks in advance and best regards!

Answer 1

Remember that by default arguments to functions are passed by value, meaning the function have a copy of the value.
When you assign to the poDataset variable in the function, you're only changing your local copy. The calling function will not see this modification. You need to pass the argument by reference:
static RASTERFUNCS_API int Open(char* rname, GDALDataset*& poDataset);

Answer 2

Joachim is spot on, but on the flip side you could pass a pointer to the pointer rather than a reference to the pointer. I won't go into the pointers vs references battle, as both are good to know and understand.
static RASTERFUNCS_API int Open(char* rname, GDALDataset **poDataset);
and
RasterFuncs::MyRasterFuncs::Open(rfileName, &poDataset);
("**" means "pointer to a pointer" and "&" means "address of")